Non-volatile memory cell

ABSTRACT

A memory cell includes an N-type well, three P-type doped regions, a first stacked dielectric layer, a first gate, a second stacked dielectric layer, and a second gate. The three P-type doped regions are formed on the N-well. The first dielectric stack layer is formed on the N-type well and between the first doped region and the second doped region from among the three P-type doped regions. The first gate is formed on the first stacked dielectric layer. The second stacked dielectric layer is formed on the N-type well and between the second doped region and the third doped region from among the three P-type doped regions. The second gate is formed on the second stacked dielectric layer.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to a non-volatile memory cell, and morespecifically, to a non-volatile memory cell having two transistors and amethod for operating it.

2. Description of the Prior Art

Non-volatile memory stores data even without a power supply so that itis widely used in various portable electronic products such as personaldigital assistants (PDA), mobile phones, and memory cards. In order torespond to these requirements, non-volatile memory technology aims forcompatibility with CMOS processing, low power consumption, high writingefficiency, low cost and high density. However, as non-volatile memoriesbecome smaller in size, their gate oxide layers become accordinglythinner so that stored data vanishes easily, which causes a problem inthe data storing ability of non-volatile memory.

Please refer to FIG. 1 showing a conventional memory cell 10. The memorycell 10 includes an NMOS transistor 28 and a PMOS transistor 30separated by an insulating field oxide layer 24. The NMOS transistor 28is formed on a P-type substrate 12 and includes a first floating gate32, an N⁺ source doped region 14, and an N⁺ drain doped region 16. ThePMOS transistor 30 is formed on an N-type substrate 18 and includes asecond floating gate 34, a P⁺ source doped region 20, and a P⁺ draindoped region 22. The PMOS transistor 30 is planted with a heavily dopedN-type channel stop region 38 under the second floating gate 34,adjacent to the P⁺ source doped region 20. The first floating gate 32and the second floating gate 34 are connected with a floating gateconductive line 36 so that both are kept at the same level. When writingdata into the memory cell 10, the first floating gate 32 generates acorresponding level according to a control gate voltage. At this timethe second floating gate 34 has the same level as the first floatinggate 32 because of the connection by the floating gate conductive line36; the level accelerates electrons in a depletion region between the P⁺source doped region 20 and the N-type channel stop region 38 and makesthem injecting into the second floating gate 34.

However, the conventional memory cell 10 has disadvantages as follows.First, the conventional memory cell 10 is composed of the PMOStransistor 30 and the NMOS transistor 28 so that it occupies a largerchip area. Second, the conventional memory cell 10 requires the floatinggate conductive line 36 to connect the first floating gate 32 and thesecond floating gate 34. Moreover, the field oxide layer 24 is requiredto separate the PMOS transistor 30 from the NMOS transistor 28.Therefore, the conventional memory cell 10 occupies too large chip areaand is structurally complicated, all of which increase the cost anddifficulties in the manufacturing process.

SUMMARY OF INVENTION

It is therefore a primary objective of the present invention to providea memory cell and a method for writing data there into in order to solvethe problems mentioned above.

Briefly summarized, a method for writing a memory cell includesproviding a memory cell which includes an N-type well, three P-typedoped regions formed on the N-type well, a first stacked dielectriclayer formed on the N-type well and between a first doped region and asecond doped region from among the three P-type doped regions, a firstgate formed on the first stacked dielectric layer, a second stackeddielectric layer formed on the N-type well and between the second dopedregion and a third doped region from among the three P-type dopedregions, and a second gate formed on the second stacked dielectriclayer; applying a common voltage to the N-type well, the third dopedregion, and the second gate; applying a voltage less than the commonvoltage to the first gate in order to erase charges stored in the firststacked dielectric layer; applying a first voltage to the first gate anda second voltage larger than the first voltage to the second gate inorder to conduct respectively P-type channels between the first dopedregion and the second doped region, and the second doped region and thethird doped region; applying a voltage larger than the second voltage tothe N-type well and the first doped region; and applying a voltage lessthan the second voltage to the third doped region in order to injectchannelhot holeinduced hot electrons in the P-type channel between thesecond doped region and the third doped region.

The present invention further provides a memory cell including anN-well, three P-type doped regions formed on the N-type well, a firststacked dielectric layer formed on the N-type well and between a firstdoped region and a second doped region from among the three P-type dopedregions, a first gate formed on the first stacked dielectric layer, asecond stacked dielectric layer formed on the N-type well and betweenthe second doped region and a third doped region from among the threeP-type doped regions, and a second gate formed on the second stackeddielectric layer.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a conventional memory cell.

FIG. 2 illustrates a memory cell according to the present invention.

FIG. 3 illustrates a memory module composed of the memory cells shown inFIG. 2.

FIG. 4 illustrates the writing operation of the memory cell.

FIG. 5 illustrates the writing operation of the memory module.

FIG. 6 illustrates the reset operation of the memory cell.

FIG. 7 illustrates the reset operation of the memory module.

DETAILED DESCRIPTION

Please refer to FIG. 2 showing a memory cell 40 according to the presentinvention, and FIG. 3 showing a memory module 60 composed of the memorycells 40 shown in FIG. 2. The memory cell 40 includes a P-type substrate42, an N-type well 44, three P-type doped regions 46, 48, 50, a firststacked dielectric layer 52, a first gate 54, a second stackeddielectric layer 56, and a second gate 58. The N-type well 44 is formedon the P-type substrate 42, the three P-type doped regions 46, 48, 50are formed on the N-type well 44, the first stacked dielectric layer 52is formed on the N-type well 44 and between the first doped region 46and the second doped region 48 from among the three doped regions, thefirst gate 54 is formed on the first stacked dielectric layer 52, thesecond stacked dielectric layer 56 is formed on the N-type well 44 andbetween the second doped region 48 and the third doped region 50 fromamong the three doped regions, and the second gate 58 is formed on thesecond stacked dielectric layer 56.

The first gate 54 and the second gate 58 can be polysilicon layers,policide layers, or metal layers. Both the first stacked dielectriclayer 52 and the second stacked dielectric layer 56 include respectivelyfirst silicon dioxide layers 522, 562; charge storage layers 524, 564;and second silicon dioxide layers 526, 566, wherein the charge storagelayers 524, 564 can be composed of either silicon nitride (Si₃N₄) orsilicon oxynitride (Si_(x)N_(y)O_(z)). The first gate 54, the firststacked dielectric layer 52, the first doped region 46, and the seconddoped region 48 form a select transistor 62 for turning on and off thememory cell 40. The second gate 58, the second stacked dielectric layer56, the second doped region 48, and the third doped region 50 form astorage transistor 64 for providing the memory cell 40 to store data. Inpractice, the memory cells 40 are aligned to create the memory module60. As shown in FIG. 3, the memory module 60 is composed of a pluralityof memory cells 40. These memory cells 40 are aligned in the form of anNOR-array, wherein the first gates 54 of the memory cells on the samecolumn are connected to the same select gate lines (SG0, SG1, SG2), thesecond gates 58 of the memory cells on the same column are connected tothe same word lines (WL0, WL1, WL2), the third doped regions 50 of thememory cells on the same row are connected to the same bit lines (BL0,BL1, BL2), and the first doped regions 46 of all the memory cells areconnected to the same source line (SL).

Please refer to FIG. 4 showing the writing operation of the memory cell40. The writing operation of the memory cell 40 is to inject electronsinto the charge storage layer 564 of the storage transistor 64 bychannel-hot-hole-induced-hot-electron injection. Before writing datainto the memory cell 40, turn on the select transistor 62 of the memorycell 40; that is to apply 0V to the N-type well 44 and −5V to the firstgate 54 so that a P-type channel between the first doped region 46 andthe second doped region 48 is conducted. When writing data into thememory cell 40, apply 1V to the second gate 58, 0V to the first dopedregion 46, and −5V to the third doped region 50. Since the P-typechannel between the first doped region 46 and the second doped region 48is conducted and the first doped region 46 and the second doped region48 have the same level, the operation of the storage transistor 64 is ina saturation region, meaning that holes in a P-type channel between thesecond doped region 48 and the third doped region 50 are likely to beaccelerated by an electric field to obtain a high energy and collidewith the N-type well 44 to generate electron-hole pairs. The electronsgenerated after collision are attracted by the voltage applied to thesecond gate 58 and injected into the charge storage layer 564 of thestorage transistor 64.

Please refer to FIG. 5 showing the writing operation of the memorymodule 60. The writing operation of the memory module 60 is similar tothat of the memory cell 40; that is to apply the voltages to the firstgate 54, the second gate 58, the first doped region 46, and the thirddoped region 50 of the memory cell 40 correspondingly to the select gatelines (SG0, SG1, SG2), the word lines (WL0, WL1, WL2), the source line(SL) and the bit lines (BL0, BL1, BL2). In FIG. 5, all the operationvoltages in FIG. 4 are shifted by 5V in order to achieve positivevoltage operation. When writing data into the memory module 60, firstselect a memory cell to be written 70, then apply 5V to the N-type well,0V to the select gate line (SG1) connected to the memory cell to bewritten 70, 5V to the select gate lines (SG0, SG2) not connected to thememory cell to be written 70, 4V to all the word line (WL0, WL1, WL2),5V to the source line (SL), 0V to the bit line (BL1) connected to thememory cell to be written 70, and 5V to the bit lines (BL0, BL2, BL3)not connected to the memory cell to be written 70. When writing datainto the memory module 60, the P-type channels of the storage memories64 of all the memory cells are conducted. Although the selecttransistors 62 of the memory cells on the same column with the memorycell to be written 70 are also turned on, there is no write interferenceto the storage memories of these memory cells, since the word lines andthe bit lines are applied with the same voltage. However, the memorycells on the same row with the memory cell to be written 70 are appliedwith 5V select gate line voltage, 4V word line voltage and 0V bit linevoltage so that band-to-band tunneling effect may occur. But at thistime the 4V word line voltage conducts the storage transistor of thememory cell 72, and the 5V select gate line voltage turns off the selecttransistor of the memory cell 72 so that band-to-band tunneling effectoccurs on the select transistor of the memory cell 72, meaning thatelectrons are injected into the charge storage layer of the selecttransistor by band-to-band tunneling. Therefore, write interference tothe storage transistor of the memory cell can be prevented by the selecttransistor of the memory cell 72.

Please refer to FIG. 6 showing reset operation of the memory cell 40,and FIG. 7 showing reset operation of the memory module 60, both ofwhich are according to the present invention. The memory module 60 maycause write interference to the memory cells 72 on the same row with thememory cell to be written 70 when writing, but since the selecttransistor of the memory cell 72 instead of the storage transistor ofthe memory cell 72 receives write interference, the accuracy of datastorage is not influenced. Whenever there are electrons injected intothe select transistor of the memory cell 72, the threshold voltage ofthe select transistor to generate the P-type channel is changed.Therefore, the present invention provides a method for resetting thememory cell 40 and the memory module 60, which is to erase electronsinjected into the select transistor due to write interference. As shownin FIG. 6, the memory cell 40 operates a reset operation byFowler-Nordheim (FN) tunneling; that is to apply 6V to the N-type well44, the first doped region 46 and the second gate 58, and −4V to thefirst gate 54 so that the first stacked dielectric layer 52 of theselect transistor 62 has a cross voltage of 10V, in order to erase theelectrons limited within the charge storage layer 524 of the firststacked dielectric layer 52. The third doped region 50 just needs to befloated during the reset operation. The reset operation of the memorymodule 60 is similar to that of the memory cell 40; that is to apply thevoltages applied to the first gate 54, the second gate 58, the firstdoped region 46, the N-type well 44 and the third doped region 50 of thememory cell 40 correspondingly to the select gate lines, the word lines,the source line, the N-type well and the bit lines, as shown in FIG. 7.

In contrast to the prior art, the memory cell according to the presentinvention includes the storage transistor serially connected to theselect transistor, and both of which are PMOS transistors. Therefore thememory cell is structurally simple, occupies less chip area, and can bemanufactured by a general CMOS process. In the memory cell, data isstored in the storage transistor having the stacked dielectric layer byinjecting electrons into the charge storage layer of the storagetransistor by channel-hot-hole-induced-hot-electron injection. And afterthe memory cells are arranged for the memory module, the selecttransistors of the memory cells can protect the storage transistors fromwrite interference between the memory cells. Moreover, the selecttransistor of the memory cell also has the stacked dielectric layer.When the memory cell receives write interference, the electrons areinjected into the stacked dielectric layer of the select transistor tochange the threshold voltage of the select transistor. Therefore thepresent invention further provides a method for writing data into thememory cell and a method for resetting the select transistor of thememory cell and erasing the electrons stored in the select transistorbefore writing data into the memory cell.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A method for writing a memory cell comprising: providing a memorycell comprising an N-type well, three P-type doped regions formed on theN-type well, a first stacked dielectric layer formed on the N-type welland between a first doped region and a second doped region from amongthe three P-type doped regions, a first gate formed on the first stackeddielectric layer, a second stacked dielectric layer formed on the N-typewell and between a second doped region and a third doped region fromamong the three P-type doped regions, a second gate formed on the secondstacked dielectric layer; applying a common voltage to the N-type well,the first doped region and the second gate; applying a voltage less thanthe common voltage to the first gate in order to erase charges stored inthe first stacked dielectric layer; applying a first voltage to thefirst gate and a second voltage larger than the first voltage to thesecond gate, in order to conduct respectively P-type channels betweenthe first doped region and the second doped region and the second dopedregion and the third doped region; applying a voltage larger than thesecond voltage to the N-type well and the first doped region; andapplying a voltage less than the second voltage to the third dopedregion in order to inject channel hot hole induced hot electrons intothe second stacked dielectric layer formed on the N-type well andbetween the second doped region and the third doped region.
 2. Themethod of claim 1 wherein each stacked dielectric layer comprises: afirst silicon dioxide layer formed on the N-type well; a charge storagelayer formed on the first silicon dioxide layer; and a second silicondioxide layer formed on the charge storage layer.
 3. The method of claim2 wherein charge storage layer is composed of silicon nitride (Si₃N₄).4. The method of claim 2 wherein charge storage layer is composed ofsilicon oxynitride (Si_(x)N_(y)O_(z)).